1. Field of the Invention
The present invention relates generally to earth tiebacks and, more particularly, to tendon rods protected from corrosion and adapted to be permanently left in place and a method of installing such a tieback in the ground.
2. Description of the Prior Art
Earth tiebacks, also generally referred to as earth anchors, are often used in the construction industry to support or anchor various structures in the ground. For example, they are used to support retaining walls bordering highways or to support excavation sheeting to prevent cave-ins which would otherwise endanger lives and property. Such tiebacks generally comprise a steel tendon rod installed in the ground and secured at its outer end by an anchor head structure to an excavation sheeting system or other structure to be supported. A concrete anchor is formed around the inner end of the tendon to distribute to the surrounding soil forces applied to the tendon. An example of a typical tieback is illustrated in U.S. Pat. No. 3,490,242.
When constructing permanent structures, it is often necessary to leave the tiebacks in place rather than remove them, thus giving rise to a need for a tieback which does not corrode when exposed to ground water or the like. Anchor tendon rods are normally manufactured from high strength prestressing steel which corrodes very rapidly when not protected.
U.S. Pat. No. 3,753,354 is one example of a corrosion protected tieback comprising a tubular plastic pipe placed around the tendon. The complex structure of this patent is expensive and requires great care in manufacture. Moreover, it requires a large diameter anchor hole which necessitates expensive drilling equipment and techniques. Other conventional attempts at corrosion protection using tubular plastic pipe over the unbonded zone of the tendon leave open space between the pipe and the tendon susceptible to the influx of water with resultant corrosion. It is difficult to securely seal the ends of the pipe to prevent water from reaching the enclosed tendon thus increasing the complexity and cost in manufacturing the tieback.
It has also been known to install plastic or metal pipe over tieback tendons in the unbonded zone and to fill the space between the pipe and the tendon with grease or other corrosion barrier. However, it is difficult to uniformly inject the grease around the tendon, particularly a ribbed tendon and the anchor zone is protected only by the grout. Further, the expense is increased due to the added cost of the grease and its injection. In addition, the diameter of the tieback is significantly enlarged requiring that a larger hole be drilled or driven into the ground to receive the tendon. It necessarily becomes more expensive to install such tiebacks since bigger and more powerful drilling equipment must be employed.
Earth tiebacks have also been protected from corrosion by a coating of epoxy applied by hand painting or dipping. However, such coatings have been unsuccessful because of nonuniformity of application and vulnerability to abrasion, chipping cracking. Another method of protecting anchor rods generally is that shown in U.S. Pat. No. 3,675,381 to Watson who uses a coating of an asphalt mastic to electrically insulate an anchor rod from a metallic anchor portion and the surrounding soil to prevent electrolytic corrosion. However, the application of the mastic to the anchor rod of Watson is difficult and time consuming due to its pasty nature and also requires the use of a sleeve around the mastic coating to protect the clothes of any person who might come into contact with the coated rod.